Patrice Le Moal

Dynamic electro-thermo-mechanical modelling of a U-shaped electro-thermal actuator

In this paper, we develop original analytical electro-thermal and thermo-mechanical models for the U-shaped electro-thermal actuator. The dynamics of the temperature distribution and displacement are obtained as a direct relationship between the systems dimensions, material properties and electrical input. The electro-thermal model provides an exact solution of the hybrid partial differential equations that describe the electro-thermal behaviour for each of the actuators three connected arms. The solution is obtained using a new calculation method that allows the representation of an integrable function by a hybrid infinite sum of sine and cosine functions. The displacement at the actuators tip is then calculated using a quasi-static model based on the superposition and virtual works principles. The obtained temperature and displacement solutions are then discussed and compared with finite element method simulations via ANSYS® and experimental results. Comparisons showed good agreement making the proposed modelling a reliable alternative which paves the way for improving the design and optimising the dimensions of U-shaped micro-actuators.

Toward Standard Method for Microelectromechanical Systems Material Measurement through On-Chip Electrostatic Probing of Micrometer Size Polysilicon Tensile Specimens

Polysilicon deposited by low pressure chemical vapor deposition is the most widely used structural material for surface-micromachined microelectromechanical systems. Thus, investigations are still needed in order to identify polysilicon mechanical characteristics as a function of the deposition conditions. The following work focuses on new micrometer size monolithic test cells combining polysilicon tensile specimens and ultra-high driving force electrostatic actuators fabricated from a common process flow. The proposed approach allows intrinsic material properties such as polysilicon tensile fracture to be measured through electrostatic probing, using on-chip micrometer size uniaxial tensile test cells.

Modeling and Stress Analysis of a Pre-Shaped Curved Beam: Influence of High Modes of Buckling

In this paper, we investigate the effect of high modes of buckling on the mechanical behavior of a pre-shaped curved beam. In a first stage, the presented modeling develops further the snapping forces solution and bistability conditions in order to include high modes of buckling. In a second stage, we develop the analytical solution of the stresses inside the beam during deflection between the two sides of buckling. The buckling with or without mechanical conditions on antisymmetric modes, the force characteristics, bistability conditions and stresses are described in this paper based on mathematical approach in order to provide a clear physical understanding of the curved beam behavior and its design parameters. The accurate knowledge of the design parameters is important in order to achieve the best integration of the curved beam in a complete microstructure. The analytical results are compared with and without considering high modes of buckling and have shown to be in excellent agreement with FEM simulations. The results show the importance of the high modes in calculating stresses and snapping forces.

Mechanical stop mechanism for overcoming MEMS fabrication tolerances

A mechanical stop mechanism is developed in order to compensate MEMS fabrication tolerances in discrete positioning. The mechanical stop mechanism is designed to be implemented on SOI wafers using a common DRIE etching process. The various fabrication tolerances obtained due to the etching process are presented and discussed in the paper. The principle and design of the mechanism are then presented. Finally, experiments on microfabricated positioning prototypes show accurate steps unaffected by the fabrication tolerances.

Analysis of the dynamic behavior of a doped silicon U-shaped electrothermal actuator

The dynamic behavior of a U-shaped electrothermal actuator is investigated in this paper based on experimental findings and FEM simulations. Few previous works are found in the literature that have addressed the dynamic response of U-shaped actuators. A lack of data on the transient displacement of the actuator in the heating and cooling cycles is noted. Experiments are made in order to characterize the dynamic response of the actuator. The actuators are micro-fabricated on doped SOI wafers and the displacement is recorded with a high speed camera. FEM simulations are in good agreement with the experimental findings. Simulations and experiments show that the shape of displacement follows evolution of the temperature distribution inside the actuator. The influence of the dynamic behavior on the control of the actuator is finally discussed.

Design optimization of bistable modules electrothermally actuated for digital microrobotics

This article deals with the optimization of the main element of a digital microrobot, which is a bistable module. the bistable module consists of curved beams and electrothermal actuators. A design study of the curved beam and the electrothermal actuator is presented in order to achieve the limits in line with miniaturization. Finally, An optimization for the dimensions is proposed respect to microfabrication and elastic limits.

On-Chip Investigation of Torque/Speed Characteristics on New High-Torque Micrometer-Size Polysilicon Electrostatic Actuators

In this study, we investigate new-generation high-torque/low-speed electrostatic micromotors as well as original on-chip testing methods for the mechanical characterization of silicon-based microactuators. Torque/speed characteristics of micrometer-size electrostatic actuators are measured for the first time using real-time elastic braking video analysis of polysilicon rotors that are monolithically coupled with elastic mechanical sensors. Loading characteristics measured through on-chip electrostatic probing indicate the potentialities of emerging actuator design methodologies involving frictional stator/rotor contact interactions on the micrometer scale.

Direct-Drive Electrostatic Micromotors Using Flexible Polysilicon Rotors

This paper is devoted to the design, the fabrication and the characterization of a new generation of direct-drive silicon based electrostatic micromotors. The following work points out the main torque limitations that occur in the design of conventional electrostatic actuators using electrostatic field interactions. Stator/rotor contact interactions are then considered, in order to provide larger external torque. Distributed motion systems are also investigated, because of significant driving forces that are expected through cooperative arrayed microactuators. According to this analysis, annular-type micromotors having a (500 Atm external diameter are proposed. The motor actuation involves stator/rotor contact interactions by using a flexible polysilicon rotor which combines arrayed elementary bending actuators. Moreover, the rotor integrates a self-adjusted torque sensor that allows the driving torque to be directly measured. Experimental characteristics point out the relevance of the proposed design rules. Unusual driving torque, one thousand times higher than ones obtained using conventional electrostatic wobble motors, have been already measured. Rotation speed ranging from 0.001 up to 750 rpm is also reported, according to the driving voltage frequency which was tuned from 1 Hz up to 500 kHz.

Investigation of Output Mechanical Power Limits on High-Torque Electrostatic Actuators Using High-Frequency Complementary Metal Oxide Semiconductor (CMOS) Camera Combined with Image Processing Software

The following work investigates output mechanical power limits of electrostatic micromotors using stator/rotor contact interactions. Loading characteristics of micrometer size motors monolithically coupled with elastic polysilicon torque sensors are measured through an high-frequency complementary metal oxide semiconductor (CMOS) camera combined with a specific image processing software. Short duration video sequences recorded during the rotor braking are analyzed by tracking of a target pixel attached to the tested devices. Output mechanical power limits are measured through electrostatic probing on the wafer level within a few milliseconds. Experiments point out an unusual mechanical power per mass unit which is as high as 100 W/gr, considering driving frequencies on the order of 100 kHz.

Electromechanical characterization of screen-printed PZT thick films

Thick soft and hard-PZT films have been fabricated on different geometries of Alumina substrates using a screen-printing technique. Three mechanical characterization tests were set up to determine Youngs modulus of the films: dynamic, quasi-static and nanoindentation tests. A Youngs modulus around 52+/- 7 Gpa was calculated. The piezoelectric coefficient d31 was also investigated and is found to be close to -33pC/N. The fabricated films showed also good performances when used in damping control.